In March 2015, the American space program sent astronaut Scott Kelly to live on a space station for a whole year, where he will not feel gravity as the space station orbits Earth. Scott Kelly’s twin brother, Mark, stayed at home. Scientists then plan to study the twins’ body chemistry, genetics, sight, and other data. This way, scientists can find out how living with and without gravity affects two identical people. What will most likely be a limitation for other scientists trying to recreate this study?

There are ethical controversies over testing human subjects.
There are probably very few astronauts with twin siblings.
There are too many complexities involved in studying body chemistry, genetics, and sight.
There is probably not enough technology to keep someone in space for an entire year.